A channel emulator is a specialized piece of electronic test equipment that emulates propagation of radio-waves based on well-defined environmental conditions. Channel emulators have been available for many years. Examples include the Spirent SR5500 and the Elektrobit Propsim. A channel emulator can emulate a reproducible set of environmental conditions that allows the verification of radio performance, as well as comparative evaluation of different radios under identical (emulated) propagation conditions. Traditional channel emulators were designed for conventional radios, now referred to as single input single output (SISO) radios. A SISO channel emulator models a single fading channel between a transmitter and a receiver by emulating multipath and Doppler fading in a multitude of predefined environmental conditions described by channel models. Multipath is a phenomenon whereby a transmit signal reflects from multiple surfaces and arrives at the receiver in the form of a sum of multiple delayed versions of itself. Multiple versions of the same transmit signal add together either constructively or destructively, resulting in time-variable signal attenuation known as multipath fading. Mobile reflectors or mobile radios introduce time-variable Doppler fading, which is a function of velocity of the reflectors or radios.
A channel emulator typically models both multipath and Doppler fading according to established statistical channel models. The delay spread of multipath reflections is a function of the size of the physical environment being modeled. Delay spread is narrower for small spaces (e.g. a small office) and wider for large spaces (e.g. outdoor environments). Doppler fading exhibits a higher frequency spread (rate of change) for fast-moving reflectors (e.g. high speed train) and a lower frequency spread for slow-moving reflectors (e.g. walking people).
A typical channel emulator downconverts the RF signal transmitted by a device under test (DUT), digitizes this signal into a stream of in-phase and quadrature (IQ) samples and mathematically processes the IQ streams according to a selected multipath and Doppler fading model. The resulting signal is then upconverted and coupled into the receiving device under test (DUT).
Modern 2-way data communications devices, including 802.11n and 3rd generation partnership project (3GPP) long term evolution (LTE) radios, use multiple input multiple output (MIMO) technology. A MIMO radio is composed of multiple receive and transmit chains operating in phase lock and capable of sophisticated radio transmission techniques designed to increase data throughput and operating range of wireless links. A MIMO link is typically described as an N×M link, where N is the number of transmit chains in a transmitting radio and M is the number of receive chains in the receiving radio. In a MIMO link each of the M receive chains detects signals from all of the N transmit chains. Thus, a MIMO channel emulator must model N times M fading channels (as compared to a SISO emulator that models only one fading channel). Each fading channel is typically implemented as a tapped delay line (TDL) structure, as shown in FIG. 2. In prior art channel emulator implementations the number of fading channels grows exponentially with the number of ports. For example a 2×2 MIMO channel emulator implements 4 fading channels, as shown in FIG. 3. A 4×4 MIMO channel emulator implements 16 fading channels, as shown in FIG. 4.
A unidirectional MIMO channel emulator for an N×M MIMO system has N receive and M transmit ports. The transmitting DUT connects to the N receive input ports of the channel emulator and a receiving DUT connects to the M transmit output ports. If the channel being modeled is bidirectional, a channel emulator typically duplicates the circuitry in the forward and reverse direction to accommodate 2-way transmission between the DUTs, as shown in FIG. 5. The DUT RF ports are typically bidirectional with RF circulators separating transmit from receive signals.
In prior art implementations the amount of computing hardware required to implement a multiport channel emulator grows exponentially with the number of ports because there is a full mesh of computationally intensive fading channels interconnecting any port to all the other ports. Channel models, specifying the time-variable tap coefficients for the TDL multipliers and correlation of these coefficients, are defined by industry standards from organizations such as 3GPP and IEEE. Channel models can also be defined by end users or recorded for real environments using channel sounding techniques.
Modeling channel impairments requires real-time high performance computing hardware, particularly for high order MIMO systems where the number of fading channels scales exponentially with the number of ports in the system. For this reason current channel emulators are expensive and bulky. Most emulators on the market today are derived from earlier and much simpler SISO emulators that only model a single fading channel. Prior art emulators use large and costly Field Programmable Gate Arrays (FPGAs) such as Altera Stratix and Xilinx Virtex devices. These components usually add tens of thousands of dollars to the channel emulator cost and require several processing boards and interfaces that result in a large system size. The cost of these components was justifiable for a single-fading-channel SISO topology in a relatively young low-volume wireless market. Presently, though, with high port count MIMO channel emulator requirements, the now mature and much bigger wireless industry requires a scalable and less expensive architecture for channel emulation. Prior art channel emulator technology, if scaled to support modern high order MIMO systems, can cost as much as $1-2 million dollars. The high cost of channel emulators prohibits their widespread use. A company or research institution may have to forgo testing over realistic channels, or it may resort to time-sharing a small number of expensive channel emulators.